In the broadest sense, an electronic interconnect is any medium which provides a path for the flow of electrons from one electrical component to another. Interconnects vary widely in their use and function. Often, they provide mechanical strength and a means for heat dissipation as well as electrical conductivity. For example, thin gold or aluminum wires are commonly used to connect integrated circuit (IC) chips to leadframes or substrates. Such interconnects act merely to provide an electrical path to and from the silicon. Solder joints, on the other hand, provide mechanical adhesion of surface mount packagas to boards or substrates in addition to functioning as electrical conduits. Further, die attach media can be configured to act as paths for heat flow as well as providing electrical connection and mechanical adhesion between the back side of the chip and the package or substrate.
With the advent of Surface Mount Technology (SMT), fine pitch packages (25 mil pitch) and items such as Tape-Pak (trademark of Intel Corporation) and Multistrate Module have placed increasingly stringent requirements upon interconnect media. Conventional screen printed solder lands have beer successfully used for surface mounting of plastic leaded chip carriers (PLCC's), available in 50 mil center packages, for example. However, extension of the basic technology to surface mount of 25 mil center (fine pitch) packages has been unsatisfactory. Significant rework efforts are necessary to remove solder bridges between leads and opens between package leads and solder lands following the IR solder flow process.
Currently, polymeric materials filled with metal powders are being investigated as alternatives to conventional soldering techniques. These materials include traditional filled epoxy composites as well as newer systems based on solvent soluble, thermoplastic resins. Filled organics offer the advantage over solder in that they maintain higher viscosities during the bonding process. This feature allows the organic interconnect to maintain its as-screened shape more readily than solders which liquify completely during reflow.
Thus, the prior art has provides a number of techniques for forming electrical interconnects, but all have limitations or disadvantages.